Osteoporosis is a metabolic bone disease characterized pathologically by an absolute decrease in the amount of bone, and clinically by increased susceptibility to fractures. Riggs et al., N. Engl. J. Med. (1986), 314:1676; Rusbach et al., In: Textbook of Endocrinology, Ed(s) Williams, (1981), p. 922; Riggs, In: Cecil Textbook of Medicine, Ed(s) Wyngaarden et al., (1985), p. 1456; Riggs et al., Am. J. Med., (1983), 75:899.
In post-menopausal women, estrogen deficiency has been identified as a major predisposing factor. Recent studies in normal women ages 20 to 88 years indicate, however, that substantial bone loss from the axial skeleton occurs gradually in the decades before estrogen deficiency ensues at menopause. Riggs et al., J. Clin. Invest., (1986), 77:1487. According to Riggs et al., " . . . factors in addition to estrogen deficiency must contribute to the pathogenesis of involutional osteoporosis in women because about half of overall vertebral bone loss occurs premenopausally." (Id.).
Calcium deficiency is believed to be one of those additional factors. Riggs, In Cecil Textbook of Medicine, Id.; Nordin, (1985), Lancet 2:720; Fujita, (1986), 12:49; Heaney, In: Osteoporosis II Ed(s), Bonzel, (1979), p. 101; and Heaney, (1982), J. Lab. Clin. Med. 100:309. Three conditions, in turn, have been identified as predisposing to calcium deficiency: suboptimal calcium intake, subnormal intestinal calcium-absorptive ability and normal or above average protein intake, Heaney, In Osteoporosis II, Id.; Heaney et al., (1982), Am. J. Clin. Nutr. 36:986.
More than 50% of women in the United States ingest less calcium than is recommended (Recommended Dietary Allowance [RDA]) by the National Research Council. Committee on Dietary Allowances, Food and Nutrition Board: Recommended Dietary Allowances, 9th Ed. (1980). In post-menopausal women, the difference between actual and optimal calcium intake is underestimated, since the recommended intakes do not take into consideration that intestinal calcium absorptive ability ordinarily declines with age. Heaney et al., Id.; Avioli, et al., (1965), J. Clin. Invest., 44:1960; Bellamore et al., (1970), Lancet, 2:535; Alevizaki, et al., (1973), J. Nucl. Med. 14:760; and Gallagher et al., (1979), J. Clin. Invest. 64:729. At sub-RDA and RDA levels of calcium intake, calcium balance (calcium intake less calcium excretion) is negative in apparently normal adults over a wide span of ages. Lutz, (1984), Am. J. Clin. Nutr. 39:281.
An increasing demand on body calcium stores is imposed by increasing dietary protein, which increases urinary excretion of calcium. Lutz, Id.; Schuette, et al., (1982), J. Nutr. 112:338; Lutz, et al., (1981), Am. J. clin. Nutr., 34:2178; Hegsted, et al., (1981), J. Nutr. 111:553; Schuette, et al., (1980), J. Nutr. 110:305; Allen, et al., (1979), 32:741; and Margen, et al., (1974), Am. J. Clin: Nutr. 27:584. Intestinal absorption of calcium fails to increase commensurately with protein-induced calciuria, hence external calcium balance becomes negative. Lutz, Id.; Schuette, et al., (1982), Id. In combination, low dietary intake of calcium and high dietary intake of protein result in greater calcium deficiency than does either condition alone. Lutz, Id.; and Rekha, et al., 1974, J. Nutr. 104:695.
It has been reported that substitution of sodium bicarbonate for an equivalent amount of sodium chloride in the diet could reverse the negative calcium balance that occurs in women (including post-menopausal women) who are in negative calcium balance on a regimen of average calcium and moderately increased protein intake. Lutz, Id. The bicarbonate-induced improvement in calcium balance occurred with unchanged calcium intake. The observations of Lutz introduce the component of acid-base balance as a potential regulatory factor in calcium balance in normal subjects and raises important questions concerning the potential role of acid-base factors in the pathogenesis of involutional osteoporosis.
More recently, it has been reported that potassium bicarbonate, but not sodium bicarbonate, reduces urinary calcium excretion and improves calcium balance in healthy young men; specifically, the external calcium balance was rendered less negative. Lemann, et al., (1989), Kidney Int. 35:688. Based upon the data obtained, it was suggested that the potassium bicarbonate may have been deposited as bone carbonate. Analysis of urinary hydroxyproline excretion did not provide evidence of reduced bone resorption, although inhibition of bone resorption could not be excluded; the measured change in calcium balance was small. Finally, the possibility that KHCO.sub.3 administration may have acted to stimulate bone formation was not evaluated. Lehmann et al., Id., at 694.
It has also been noted in the literature that the administration of other alkalinizing potassium supplements induces a decrease in urinary calcium excretion (Sakhaee, et al., (1983), Kidney Int. 24:348). In this study it was shown that sodium citrate failed to significantly reduce urinary calcium excretion, while the administration of potassium citrate was attended by a significant decline in urinary calcium levels.
It is among the objects of the present invention to provide a method which is not only useful for reducing calcium excretion and thereby improving calcium balance, but which decreases bone resorption and conserves and/or increases bone mass in both normal and osteoporotic patients.